Efficient CNG production

Compressed natural gas (CNG) could become a cheaper transportation fuel option with a technology that promises to produce the gas more efficiently.

Researchers in Oxford have discovered two base metals that could be used to remove impurities, such as mercury and sulphur, from CNG. This is typically carried out through chemisorption, a technique that removes pollutants by involving them in a chemical reaction.

Efficiency of the chemisorption process depends heavily on the composition of the catalyst which, in this case, is two base metals.

The metal catalyst, also called a chemisorbent, was designed and developed by Oxford Catalysts Group, a spin-out from the university, and will soon be tested on an industrial scale with Thai state-controlled oil and gas company PTT. Oxford Catalysts is also working with a leading company to scale up manufacture of the catalyst for commercial use.

Derek Atkinson, business development director of Oxford Catalysts, said laboratory tests at Nottingham University show the new catalyst has a much greater capacity, or ability, to take up more pollutant per unit volume, than existing chemisorbents including sulphur impregnated carbon and copper zinc materials.

The key to its improved performance lies in the chemisorbent composition — the combination of the metals used in the chemisorbent.

‘The two metals together offer some synergistic effect,’ said Atkinson. ‘If there is a natural gas stream that has mercury in it and no sulphur there will be a reaction between the metals and the mercury. The compound is then permanently withheld in the structure of the material.

‘If there’s sulphur present you get an additional reaction occurring in which you get a mercury sulphide formed and this is absorbed on to the surface of the material.’

Atkinson said this differs from existing materials that need to be treated with a sulphur-bearing material to react with impurities.

Oxford Catalysts will test the catalyst at two PTT facilities, one offshore and one onshore. It will set up reactors filled with 50 tons of its metal catalyst alongside existing reactors to predict how its chemisorbent compares with the existing material. Atkinson said: ‘These side stream reactors will be in place toward the end of this month and then we will operate them for some time and generate the data.’

The chemisorbent is expected to work for six months with streams that contain heavy amounts of mercury and several years where mercury is less concentrated.

Atkinson said not all gas streams have mercury, but some countries, including Thailand, have high levels. ‘There are two reasons you want to remove that mercury,’ he said. ‘One is because it’s a health hazard. The other reason is if you have any requirement to cool the material you need a heat exchanger — an aluminium heat exchanger — and the mercury would react with the aluminium.’

He said this has been a problem, citing the 2004 disaster at Skikda liquefied natural gas plant in Northern Algeria, where 27 people died and 80 were injured. Train 40 was the first of six liquefaction trains that exploded and it did not have mercury removal sections.

While there are many uses for natural gas, in its compressed form it is becoming popular for transportation fuels.

CNG is produced at relatively low cost and is cleaner burning than petrol or diesel. According to data from the US Department of Energy, natural gas vehicles show a 20 to 40 per cent reduction in CO emissions and 80 per cent reduction in particulate matters compared to petrol vehicles.

Siobhan Wagner

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